1. Field of the Invention
The present invention is directed to systems and methods for fabricating integrated circuit resonant devices, and particularly a process for manufacturing integrated circuit (IC) band-pass filters using micro electromechanical system (MEMS) technology on single crystal silicon-on-insulator (SOI) wafers in a manner consistent with current integrated circuit fabrication techniques.
2. Discussion of the Prior Art
Micro Electro-Mechanical Systems (MEMS) technology is currently implemented for the fabrication of narrow bandpass filters (high-Q filters) for various UHF and IF communication circuits. These filters use the natural vibrational frequency of micro-resonators to transmit signals at very precise frequencies while attenuating signals and noise at other frequencies. FIG. 1 illustrates a conventional MEMS bandpass filter device 10 which comprises a semi-conductive resonator structure 11, e.g., made of polycrystalline or amorphous material, suspended over a planar conductive input structure 12, which is extended to a contact 13. An alternating electrical signal on the 12 input will cause an image charge to form on the resonator 11, attracting it and deflecting it downwards. If the alternating signal frequency is similar to the natural mechanical vibrational frequency of the resonator, the resonator may vibrate, enhancing the image charge and increasing the transmitted AC signal. The meshing of the electrical and mechanical vibrations selectively isolates and transmits desired frequencies for further signal amplification and manipulation. It is understood that the input and output terminals of this device may be reversed, without changing its operating characteristics.
Typically, resonator filter devices 10 are fabricated by standard integrated circuit masking/deposition/etching processes. Details regarding the manufacture and structure of MEMS band-pass filters may be found in the following references: 1) C. T. -C. Nguyen, L. P. B. Katehi and G. M. Rebeiz xe2x80x9cMicromachined Devices for Wireless Communicationsxe2x80x9d, Proc. IEEE, 86, 1756-1768; 2) J. M. Bustillo, R. T. Howe and R. S. Muller xe2x80x9cSurface Micromachining for Microelectromechanical Systemsxe2x80x9d, Proc. IEEE, 86, 1552-1574 (1998); 3) C. T. -C. Nguyen, xe2x80x9cHigh-Q Micromechanical Oscillators and Filters for Communicationsxe2x80x9d, IEEE Intl. Symp. Circ. Sys., 2825-2828 (1997); 4) G. T. A. Kovacs, N. I. Maluf and K. E. Petersen, xe2x80x9cBulk Micromachining of Siliconxe2x80x9d, Proc. IEEE 86, 1536-1551 (1998); 5) K. M. Lakin, G. R. Kline and K. T. McCarron, xe2x80x9cDevelopment of Miniature Filters for Wireless Applicationsxe2x80x9d, IEEE Trans. Microwave Theory and Tech., 43, 2933-2939 (1995); and, 6) A. R. Brown, xe2x80x9cMicromachined Micropackaged Filter Banksxe2x80x9d, IEEE Microwave and Guided Wave Lett.,8, 158-160 (1998).
The reference 7) N. Cleland and M. L. Roukes, xe2x80x9cFabrication of High Frequency Nanometer Scale Mechanical Resonators from Bulk Si Crystalsxe2x80x9d, Appl. Phys. Lett, 69, 2653-2655 (1996) describes the advantages of using single crystal resonators as band-pass filters. The references 8) C. T. -C. Nguyen, xe2x80x9cFrequency-Selective MEMS for Miniaturized Communication Devicesxe2x80x9d, 1998 IEEE Aerospace Conf. Proc., 1, 445-460 (1998) and 9) R. A. Syms, xe2x80x9cElectrothermal Frequency Tuning of Folded and Coupled Vibrating Micromechanical Resonators, J. MicroElectroMechanical Sys., 7, 164-171 (1998) both discuss the effects of heat on the stability of micromechanical band-pass filters. Of particular relevance as noted in these references is the acknowledgment that the existing processes for making MEMS bandpass filters have serious drawbacks. For instance, as most resonators are made of polycrystalline or amorphous materials to simplify fabrication, there is exhibited an increase in mechanical energy dissipation which softens the natural frequency of oscillation, as noted in above-mentioned references 1)-3) . Etching polycrystalline materials does not allow for device features smaller than the polycrystalline grain size, which creates rough surfaces and prevents precise mechanical characteristics. For example, above-mentioned references 1) and 2) both detail the problems encountered when polycrystalline material is used in MEMS resonators. Additionally, in reference 7), there is described the construction of resonators made of single-crystal silicon including a description of an attempt to use complex dry-etch techniques to obtain single-crystal resonators. The reference reports such resonator structures having scalloped edges, which reduces the precision of the final mechanical performance to that of polycrystalline structures. That is, their etch-process produced surface roughness that was similar to that of polycrystalline materials.
Other attempts to use single-crystal silicon have been reviewed in reference 4), however, these attempts were made to eliminate the poor device performance when polycrystalline materials were used for construction. Most used an isotropic etches to undercut single-crystal silicon surfaces and construct resonators (and other structures). In all cases, the structures were quite large, in part to minimize the effects of surface roughness and non-parallel surfaces on the device performance. Since the devices were very large, they were useful only for low-frequency applications (below 100 MHz) , which is of limited usefulness as a communication frequency filter in the commercial band of 300-6000 MHz. A further limitation of all MEMS band-pass structures is that they are formed above the silicon surface (see references 1-9). This makes the structures incompatible with standard integrated circuit fabrication, since it prevents xe2x80x9cplanarizationxe2x80x9d. After the devices of an integrated circuit have been fabricated, the wafer enters its final processing which is called xe2x80x9cmetallizationxe2x80x9d and xe2x80x9cplanarizationxe2x80x9d. Before this step, all the devices on the wafer are isolated, and for integration they must be connected together with metal wires. In modern devices, the wiring is done as a series of layers, each containing wiring in certain directions (i.e., metallization). After each layer is deposited, the wafer surface is smoothed, i.e., is planarized so that subsequent layers of wiring may be deposited on a smooth surface. Planarization is typically done by chemical-mechanical polishing (CMP processing) or by melting a thin layer of glass over the surface. If there is a micro-mechanical device protruding up above the surface, it would be immediately destroyed by either of the above planarization processes.
Additional prior art patented devices such as described in U.S. Pat. No. 3,634,787 (1972) , U.S. Pat. No. 3,983,477 (1976) and U.S. Pat. No. 4,232,265A (1980) describe similar mechanical resonatored structures, but which are incompatible with integrated circuit processing.
For instance, U.S. Pat. No. 3,634,787 describes an electro-mechanical resonator band-pass filter device having a mechanical component consisting of a support being a unitary body of semiconductor material and having a piezoelectric field effect transducer therein. Thus, its electrical operation relies upon the piezoelectrical effect. U.S. Pat. No. 3,983,477 describes a ferromagnetic element tuned oscillator located close to a high-voltage current carrying conductor, however, as such, its electrical operation relies on the ferromagnetic effect. U.S. Pat. No. 4,232,265A describes a device for converting the intensity of a magnetic or an electromagnetic field into an electric signal wherein movable elements are made as ferromagnetic plates. Likewise, its electrical operation relies upon the ferromagnetic effect. U.S. Pat. No. 5,594,331 describes a self-excitation circuitry connected to a resonator to process induced variable frequency voltage signals in a resonant pass band and is of exemplary use as a power line sensor. Likewise, U.S. Pat. No. 5,696,491 describes a microelectromechanical resonating resonator which responds to physical phenomenon by generating an induced variable frequency voltage signal corresponding to the physical phenomenon and thus, does not lend itself to manufacture by current integrated circuit fabrication technology.
It would thus be highly desirable to construct an IC MEMS band-pass filter device in a manner consistent with current integrated circuit fabrication techniques that avoids completely or reduces significantly all of the above-described limitations.
It is an object of Lhe present invention to provide an improved IC MEMS resonator band-pass filter device of a construction that lends itself to manufacture in accordance with current IC manufacturing techniques and that overcomes the fundamental weaknesses as outlined in the above-mentioned references.
Particularly, according to one aspect of the invention, there is provided a resonatored MEMS bandpass filter device that is constructed of single-crystal silicon, eliminating the mechanical problems associated with using polycrystalline or amorphous materials. The final MEMS device lies below the silicon surface, allowing further processing of the integrated circuit, without any protruding structures. The MEMS device is about the size of a SRAM cell, and may be easily incorporated into existing integrated circuit chips. The natural frequency of the device may be altered with post-processing, or electronically controlled using voltages and currents compatible with integrated circuits.
According to another aspect of the invention, there is provided a novel resonatored MEMS bandpass filter device fabrication technique for constructing such MEMS devices in a manner compatible with current integrated circuit processing.